Override graphql task doc folder in JH

What does this MR do and why?

Pipeline contains a job that will execute Rake task gitlab:graphql:validate.

It will read the content of the file doc/api/graphql/reference/index.md, compare it with the result of Rake task gitlab:graphql:compile_docs, and report an error if not equal.

If JH adds something related to GraphQL, the doc file should be updated to pass the test.

But the doc file is located outside the jh directory, so we rewrite the directory's configuration.

After updating, the path to doc file is jh/doc/api/graphql/reference/index.md

cc EM @prajnamas